Heretofore, there has been proposed a technique of operating an engine while switching between a reduced-cylinder operation mode in which combustion is stopped in a part of a plurality of cylinders (a part of a plurality of cylinders are deactivated) and an all-cylinder operation mode in which combustion is performed in all the plurality of cylinders, depending on an operating condition of the engine (see, for example, the following Patent Document 1). Particularly, in the Patent Document 1, there is disclosed a technique of delaying the execution of fuel cut-off by a given time period after satisfaction of a given fuel cut-off condition (deceleration operation condition), and gradually retarding ignition timing until this time period elapses, wherein a retard amount of ignition timing is set differently between the reduced-cylinder operation mode and the all-cylinder operation mode.
Specifically, in the technique described in the Patent Document 1, from a viewpoint of suppressing a shock caused by a rapid change in engine torque due to the fuel cut-off, the fuel cut-off is executed after reducing the engine torque by gradually retarding ignition timing. However, when the engine is operated in the reduced-cylinder operation mode, there is a possibility that ignition timing is not adequately retarded and thereby the engine torque is not sufficiently reduced, resulting in the occurrence of a shock at start of the fuel cut-off. Therefore, in the technique described in the Patent Document 1, with a view to solving this problem, during the reduced-cylinder operation mode, the retard amount of ignition timing is increased, as compared to during the all-cylinder operation mode.